Patent GB-1515287-A (GB)
Title: Piezoelectric transducers
IPC: H04R1/40
Filed: 1974-05-30
Abstract: 1515287 Peizoelectric transducers; microphone circuits; diaphragms; windshields PLESSEY CO Ltd 14 May 1975 [30 May 1974 16 Nov 1974] 25370/74 and 49709/74 Heading H4J A piezoelectric transducer (Fig. 1) includes a multilayered diaphragm 1 comprising at least two plastics layers 1a 1b separated by an electrode. At least one of the plastics layers 1a, 1b is piezoelectric and is in an untensioned state between two electrodes 2, 3, both of which are part of the multilayered diaphragm. The diaphragm 1 is held by a support 8, 9. Two piezoelectnc layers (1a, 1b) Fig. 2 (not shown) or three piezoelectric layers (1a, 1b, 1c) Fig. 3 (not shown) may have interposed electrodes and be sandwiched between further electrodes 2,3. In practice the interposed electrodes would be formed by two electrode layers because each plastics layer would be electroded and polarized prior to assembly of the multi-layer structure. The piezoelectric plastics material is preferably polyvinylidene fluoride, polyvinyl fluoride or fluorinated ethylene propylene copolymer. The piezoelectric layers 1a, 1b (Fig. 1) are polarized such that piezoelectric flexure structures are obtained i.e. are series or parallel bimorpho depending upon whether the polarization is in the same or opposite directions. Member 1 is circular and is of thickness 5 to 500Ám, preferably 10 to 100 Ám. In another arrangement only one of the layers 1a and 1b (Fig. 1) is piezoelectric with an electrode a between the layers 1 and 1b, thereby providing a unimorph. The diaphragm is clamped between two rigid members 8 using a peripheral clamp 9. Apertures 10 are in register and may be circular or polygonal with a diameter or maximum diagonal dimension of 0À05 to 100 mm., preferably 1À0 to 15  mm. An alternative arrangement (Fig. 4) has the diaphragm sandwiched between two clamped together rigid perforated members 11. The perforations are in register and divide the diaphragm into discrete regions forming separate transducer elements electrically coupled in parallel. Alternatively (Fig. 5, not shown) one of the perforated members 11 is replaced by a rigid member (12) Fig. 5 (not shown) and has the surface in contact with electrode 3 roughened or profiled. The perforations of Figs. 4 and 5 are preferably circular or diagonal. Variations in the size of the perforations within the range 0À05 to 10 mm. can be used to effect variation in the acoustic resonance and control the response. In the arrangement of Fig. 5 variations in the surface roughness or pattern may be used in addition for the same purpose. The resonant frequency of the device may be within or outside the frequency of interest or the resonance can be smoothed out to give a desired frequency response by having a range of different sized perforations with different resonances. A microphone (Fig. 7) for use as a direct replacement of a telephone carbon granules microphone includes a piezoelectric transducer 13 including two piezoelectric plastics layers 13a, 13b polarized in opposite directions, electrically connected in series and electroded with aluminium or gold. Where a gold electrode is used an intermediate layer of nickel chromium alloy may be used to ensure good adhesion. Two clamping plates 13c, 13d have coincident apertures 13e and are of 3 mm. thick polycarbonate. An integrated circuit amplifier 15 with an associated capacitor Cl is mounted on the main body 14a of an enclosure 14 which acts as a heat sink. The front electrode may be connected to the terminal 9 and to a metal enclosure 14 to effect screening. A protective front plate 14b has a hole pattern 14c which, together with the mouthpiece of the telephone handset, acts as a protective shield. A polyester foam disc 19 is compressed to provide acoustic resistance and also acts as a windscreen. A hole 17 provides equalization of pressure variations on both sides of the diaphragm below 100 Hz. The transducer may alternatively be used in an earpiece or as an ultrasonic transmitter or receiver or hydrophone. A noise cancelling microphone (Fig. 8) has two protective plates 21, one on each side of the transducer 20, and two compressed foam discs 22 each interposed between the transducer 20 and a separate one of the plates 21. The transducer 20 includes two piezoelectric layers 20b, 20c polarized in the same direction and electrically connected in parallel. Electrodes 20d, 20e, 20f are provided. Plates 20a are of brass or metallized plastics material and protective plates 21 are of aluminium and have a single hole 23. An emitter follower circuit (Fig. 9, not shown) is described for converting the high impedance of the microphone to a value of about 500 ohms. A second order pressure gradient microphone (Fig. 10c) includes two transducers 28, one at each end of an enclosure 30 and acoustically separated by a sound proof disc 31. Holes 32 and 34 in the enclosure 30 provide sound ports for the spaces 33 and 35, Third order pressure gradient microphones (not shown) use pairs of second order gradient microphones arranged in a similar manner to the arrangement of Fig. 10c. In the microphone of 10c an apertured protective plate would be provided together with an acoustic resistance in the form of a foam disc (not shown). An alternative second order pressure gradient microphone (Fig. 11) has a cylinder 42 closed at each end and divided into two chambers 43.44 by a transducer 45 having the same structure as one of the transducers 28 (Fig. 10c). Sound ports 46 and 49 allow sound pressures to act on opposite sides of the transducer 45 to form one first order pressure gradient microphone. Similarly sound ports 47 and 49 co-operate with the transducer 45 to form a second first order pressure gradient microphone. By proper arrangement of the ports 46, 47, 48, 49 the effective force on the diaphragm is proportional to the second order of the pressure gradient. Reference has been directed by the Comptroller to Specification 1,410,822.